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NEW YORK (Reuters) - If Americans stick to their eating and exercise habits, future historians will look back on the early 21st century as a golden age of svelte.

Using a model of population and other trends, a new report released on Tuesday by the Trust for America's Health and the Robert Wood Johnson Foundation projects that half of U.S. adults will be obese by 2030 unless Americans change their ways.

The "F as in Fat" report highlights the current glum picture of the U.S. obesity epidemic, in which 35.7 percent of adults and 16.9 percent of children age 2 to 19 are obese, as the Centers for Disease Control and Prevention (CDC) reported earlier this year.

But for the first time, the report builds on state-by-state data from the CDC to project obesity rates. In every state, that rate will reach at least 44 percent by 2030. In 13, that number would exceed 60 percent.

Obesity raises the risk of numerous diseases, from type 2 diabetes to endometrial cancer, meaning more sick people and higher medical costs in the future, the report said.

It projects as many as 7.9 million new cases of diabetes a year, compared with 1.9 million new cases in recent years. There could also be 6.8 million new cases of chronic heart disease and stroke every year, compared with 1.3 million new cases a year now.

The increasing burden of illness will go right to the bottom line, adding $66 billion in annual obesity-related medical costs over and above today's $147 billion to $210 billion. Total U.S. healthcare spending is estimated at $2.7 trillion.

That projection supports a study published earlier this year in the American Journal of Preventive Medicine that found that by 2030, 42 percent of U.S. adults could be obese, adding $550 billion to healthcare costs over that period.

'A TALE OF TWO FUTURES'

As with all projections, from climate models to Charles Dickens' "A Christmas Carol," human actions can prevent the worst of the scenarios, according to health policy experts.

"This is a tale of two futures," said Jeffrey Levi of George Washington University and the executive director of Trust for America's Health. "We're at a turning point where if we don't do something now to mitigate these trends, the cost in human health and healthcare spending will be enormous."

Obesity is defined as having a body mass index (BMI) above 30. Overweight means a BMI of 25 to 29.9. BMI is calculated by taking weight in pounds and dividing it by the square of height in inches, and multiplying the result by 703. For instance, someone who is 5 feet, 5 inches tall and weighs 185 pounds (84 kg) has a BMI of 30.8.

Obesity rates among U.S. adults have more than doubled from the 15 percent of 1980. In that same time, they have more than tripled among children.

Since the CDC found that the percentage of obese children and adults was essentially unchanged between 2008 and 2010, some experts question whether the "F as in Fat" model overstates future obesity by assuming past trends continue in a straight line.

"This is a strong assumption," said economist Justin Trogdon of RTI International in North Carolina. "Recent evidence from other surveys suggest obesity rates may be leveling off."

Mathematician Martin Brown of Britain's National Heart Forum, a nonprofit group, who led development of the model, said it takes a longer view by design.

"You have to take trends over a number of years," he said. "In the age groups that matter, there just isn't much evidence of a leveling off in obesity rates."

EDUCATION AND INCOME

Obesity has long been associated with education and income. The report found that about one-third of adults without a high school diploma were obese, compared with about one-fifth of those who graduated from college or technical college.

And one-third of adults who earn less than $15,000 per year are obese, compared to one-quarter of those who earned $50,000 or more per year. The obesity-poverty connection reflects such facts that calorie-dense foods are cheap and that poor neighborhoods have fewer playgrounds, sidewalks and other amenities that encourage exercise.

As a result, many states projected to have the most obesity in 2030 do now, too. In 2011, 12 states had an adult-obesity rate above 30 percent, with Mississippi the highest at 34.9 percent. Colorado was the lowest at 20.7 percent.

The report projects that in 2030 in Mississippi, 66.7 percent of adults will be obese, as will 44.8 percent in Colorado, which will still be the thinnest state.

More surprising are projections for states such as Delaware, now ranked 19 for obesity with a rate of 28.8 percent. The model uses 1999 as a baseline, explained Brown. "So if a state had a low rate of obesity in 1999 and is fairly high now, that indicates a steep rate of increase, which we believe will not go away." Result: an obesity rate of 64.7 percent in Delaware in 2030, making it the third-most obese state.

States facing the greatest percentage increase in obesity-related medical costs are now in the middle of the pack.

New Jersey faces the largest increase in costs, 34.5 percent, as its obesity rate is projected to climb from 23.7 percent today to 48.6 percent in 2030. Eight other states could see increases of 20 percent and 30 percent, including New Hampshire, Colorado and Alaska.

Trust for America's Health sees room to change that trajectory with the right interventions.

"We have learned that with a concerted effort you can change the culture of a community, including its level of physical activity, eating habits, what foods are offered in schools, and whether families eat together," said Levi.

In New York City, for instance, obesity for elementary and middle-school students dropped 5.5 percent from the 2006-07 school year to 2010-11, thanks mostly to healthier school lunches, public health experts said.

"A lot of this is about making healthy choices easier and not mandating healthier lifestyles," Levi said.